It is conventionally known that a feature of a malignant tumor can be judged based on a measured expression level of HER2 of a malignant tumor cell. In Joensuu H., et al. (Clinical Cancer Research, vol. 9, 2003, 923-930), the expression level of the HER2 is used to predict prognosis of early breast cancer. The HER2 is a growth factor receptor having a tyrosine kinase activity, and it is generally interpreted that the malignant tumor in which HER2 is expressed at a high level involves a high risk of recurrence, while the risk of recurrence is low in the malignant tumor in which HER2 is expressed at a low level. However, even the malignant tumor undergoing the low HER2 expression level still involves some risk of recurrence, and the prognosis of the malignant tumor based on the expression level of the HER2 has failed to be very accurate.